At present, whole-wood chips or surface plank chips are cleaned, to make them suitable for use in the manufacture of pulp. Reliable measurement of the bark content is essential to the operation of chip-cleaning equipment, so that the equipment can be adjusted. Correspondingly, it is essential to the optimal operation of a debarking plant to be able to measure reliably the wood content of bark material, so that the plant can be operated as efficiently as possible, without unreasonably increasing wood waste. Finnish Patent 90918 describes a method for determining the bark/wood ratio from a sample. Naturally, this method cannot be applied to on-line operation. The basis of the patent application was the observation, made at the time, that the image of woodchip or bark material did not, as such, give a reliable result of the ratio in question, on the basis of the degree of light. This was because the dispersion of the samples was too great. Thus, according to the invention, the material was ground to a small granule size, when reliable and repeatable measurements could be made from this ground material.
Swedish publication print 466420 (FI app. 922179) describes a continuously operating method, by means of which it is possible to show the presence of bark, or determine the degree of debarking of wood/woodchips. Timber as a log is transported longitudinally through an image detection station with the aid of a CCD camera, by means of which pieces of bark on the surface of the log can be discerned. In this method, a laser bean is used, the trace left by which on the surface of the log is scanned and analyzed. A clean wood surface creates a different kind of image to a bark surface, making analysis possible. If required, the actual bark content can be calculated from the ratio of the surfaces, by using known conversion graphs. A simple surface ratio is sufficient for most purposes. It can be used as a process measurement quantity, while boundary values can be defined for it for adjustment.
Finnish patent application 933517 describes a method for determining the proportions of the differently-colored surfaces of pieces in a flow of material. According to an example, the measurement of the wood content of bark chips takes place by leading the flow of material through two image detection chambers, the first of which has a wood-colored background and the second has a bark-colored background. From the intensity of the reflection, it is possible to determine the relative proportions of both components.
All known on-line methods have the drawback of random disturbances in the image, such as shadows appearing in a flow of chips, or points of discontinuity appearing on the surface of a log.
This invention is intended to create a new kind of method and equipment, by means of which a flow of woodchips, bark material, or logs can be more accurately analyzed than previously, to determine the bark/wood ratio.